348 METAMORPHOSIS 



appears in tropical plants is an adaptation to special periods in the year, so that 

 the resting period is associated with the cold or dry season and the active period 

 with the warm or wet season. This periodicity, however, cannot be at once 

 adapted to other alternations of seasons. If plants from the southern hemi- 

 sphere be brought into such a climate as ours, they are able to bring their perio- 

 dicity into harmony with our climate only if they become thoroughly acclima- 

 tized. In the case of our indigenous flora periodicity may be altered only 

 gradually, since the plants maintain their accustomed rhythm with consider- 

 able tenacity. It has already been pointed out that we can induce the forma- 

 tion of shoots by certain stimuli, but we cannot on the other hand lengthen the 

 resting period indefinitely without injury to the plant. Manifestly, activity is 

 followed by a resting period in the plant ; conversely, activity follows of necessity 

 a period of rest, and the periodic alternation between them is maintained with 

 greater or less constancy. [KLEBS (1903) has shown that many plants which 

 have well marked resting periods under natural conditions may, by artificial 

 means, be made to grow continuously. The chief point to note in this relation 

 is that one prevents the occurrence of the arrests, which increase during the course 

 of the resting period, and that already the conditions for the development of 

 shoots are provided at the commencement of iheresting period. Although we donot 

 in the least degree question the correctness of KLEBS' s very interesting obser- 

 vations, still we must take exception to the framing of any general conclusion 

 upon them. The conditions existing in trees especially, which KLEBS has 

 scarcely at all considered, compel us to hold the view that the resting period is 

 often not to be considered as an inevitable consequence of growth. If KLEBS' s 

 view has a general application we should be able to produce a continuous leaf 

 formation in trees which exhibit periodic vegetative activity.] 



The examples we have hitherto given of daily and yearly periodicity illus- 

 trate in reality only quantitative changes in growth ; there are, however, also 

 qualitative differences, so that at different seasons different kinds of organs are 

 produced. The contrast between vegetative and reproductive organs is a case in 

 point, but even the vegetative organs themselves do not always appear in the 

 same forms. In the highest plants, for instance, we recognize a regular succes- 

 sion of scale-leaves, foliage-leaves, and bracts. These qualitative changes in the 

 productive activity of the plant stand in close relation to the quantitative types 

 treated of above, since in this case also a yearly periodicity is very well marked. 

 The factors which make plants produce leaves of dissimilar form are naturally 

 internal, inquiry into which is by no means easy. Certainly, the succession 

 of these organs is not unalterable, for we can influence the succession to a certain 

 extent. The insight which we gain thereby teaches us nothing more than 

 that correlations exist between the individual organs, disturbances in which 

 induce disturbances in the normal succession. A few examples will make this 

 clearer. The normal leafy shoot of a tree, after producing a larger or smaller 

 number of foliage-leaves, proceeds to form scale-leaves, forming a covering under 

 which the next year's shoot is constructed as a terminal bud. Again, similar 

 buds are formed in the axils of the leaves, which also commence with scale-leaves. 

 The scale-leaves have a quite different function from the foliage-leaves, and hence 

 we find them to possess a distinct form and structure. They do not possess large 

 surfaces exposed to light. Chlorophylliferous tissue, permeated with vascular 

 bundles, is wanting ; they are small, compact, and closely pressed together. 

 In their first beginnings, however, they differ in no respect, as GOEBEL (1880) 

 has shown, from foliage-leaves (Fig. 104), and they exhibit, as these do, a differen- 

 tiation into leaf -base (G) and blade (L). While in the case of the foliage-leaf it 

 is especially the blade which develops greatly, in the bud-scale it does not do so 

 as a rule, the leaf-base developing instead. If, some time in the spring, we 

 remove the foliage-leaves from a developing shoot, the leaf-organs, which would 



